Imaging and measuring the contours and features of various targets without touching them, with a probe for example, is known in the art in a variety of fields.
One basic means by which a non-contact optical system can be used to acquire three-dimensional surface information is triangulation. In plane geometry, triangulation determines the distance by observing a point on an object from two different positions separated by a known base line distance. By determining the angles from the two observations sites to a single point on the surface, the length of one side of the triangle and all of the angles are known. This information is sufficient to determine the remaining length of the sides of the triangles and the heights above the base line using trigonometry. A laser can be used to project a beam at a target from a known position and angle. The location of the projected spot is imaged using a CCD camera also fixed at a known location. From the location of the spot on the image captured by the camera, the required azimuth and elevation angles can be determined providing the necessary information relative to the location in three dimensions of that point on the target surface. Since only a single data point is collected for each CCD image processed, however, the surface must be scanned in two dimensions which results in a lengthy data collection/processing duration.
Structured light systems provide a more efficient means of collecting scanned data of the surface of an object. In such a system, a line of light or “light stripe” is projected on the surface. The line is oriented so that stripe is at a constant known azimuth angle, the critical angle that must be known for triangulation. The light fans out over a wide image of elevation angles making this unknown. The light stripe is then imaged by the CCD camera, which captures a two-dimensional image of the projected stripe on the surface. The deviation of the image stripe from a known base line provides a measurement of the azimuth angle at the camera's observation point. The elevation angle may be determined from the distance in the orthogonal direction. Because there is only one elevation angle in this geometry, the system is completely determined and the structured light provides a complete scan along one direction. The mechanical surface scanning process is thus decreased from two dimensions to one dimension and the number of CCD images which must be processed is reduced from n2 to n for a square scan. Scanning is typically accomplished by rotating the specimen while holding the optical system in a fixed position.
In U.S. Pat. No. 4,737,032, incorporated herein by this reference, the contour line formed when the light stripe intersects the target is viewed from two sides but then both of these images are combined by a beam splitter so that the camera only sees one image. Optical alignment of the various optical components, however, is critical in this design and thus the resulting system is not very robust and is difficult to use in factory settings and/or by non-skilled technicians and operators. Any physical disturbance, such as shock, vibration, or thermal cycling, can cause one or more optical components to shift misaligning the system. This misalignment will produce erroneous reading by the system and which can only be corrected by a trained technician with specific skills with optics and optical systems. The lack of robustness in the design of the '032 patent limits its applicability.